Conventionally, there is a PWM control technique, for example, for a motor drive circuit. In the PWM control technique, a command voltage signal for commanding a duty ratio is inputted; an output PWM signal with a predetermined frequency is generated based on the command voltage signal; and a load apparatus is controlled by the output PWM signal.
In a drive circuit to which the PWM control technique is applied, especially in a drive circuit for driving a motor, when a coil current flowing through a coil of the motor exceeds a predetermined threshold, an internal PWM signal G is lowered from an H level to an L level to generate an output PWM signal so that an overload current does not flow through the motor.
However, in a conventional drive circuit, if the duty ratio is set to 100% in order to maximize output of a motor, the H level of the internal PWM signal is maintained until the coil current of the motor exceeds the predetermined threshold, and a frequency may be lower than the predetermined frequency.
When the frequency of the internal PWM signal becomes lower than the predetermined frequency, a frequency of the output PWM signal also becomes lower than the predetermined frequency. The motor driven by the output PWM signal having a lowered frequency may cause, for example, noise by an electromagnetic sound, or vibration, noise and the like by increase in a current ripple.